Quote:
Originally Posted by kalee20
Agree! Overall voltage gain of the cascode is gm (lower) x load resistor (upper). The gm of the upper device does not feature in the calculation.
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The best analysis of valve Cascode I have seen was done by Millman & Halkias where they point out that the voltage gain of the Cascode circuit is:
Gain = -(u(u+1)R)/(R + (u+2)rp).
rp is the plate resistance and R the load resistance.
And as pointed out in a post above, if (u+2)rp >>R and if u>>1 then the gain is simply:
Gain = A1 = -uR/rp = -gmR
Also, they outline what happens when you apply a variable voltage to the grid of the upper valve and then the lower valve acts as an impedance of magnitude rp in the cathode of the upper valve and the voltage gain, A2 for the signal applied to the upper valve is:
A2 = -uR/(R+ (u+2)rp)
So then if you apply two sinusoidal signals V1 and V2 to the lower and upper grids respectively, the output voltage, Vo is:
Vo = A1V1 + A2V2.